<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
  </head>
  <body>
    <div id="root">
      姓：<input type="text" v-model="firstName" /><br />
      名：<input type="text" v-model="lastName" /><br />
      全名：<span>{{fullName}}</span>
    </div>

    <script>
      const vm = new Vue({
        el: "#root",
        data: {
          firstName: "张",
          lastName: "三",
          fullName: "张-三",
        },
        // computed: {
        // //   a() {
        // //     return this.x + this.y;
        // //   },
        // },
        watch: {
          firstName(cur) {
            setTimeout(() => {
                this.fullName = cur + "-" + this.lastName;
            }, 1000)
          },
          lastName(cur) {
            this.fullName = this.firstName + "-" + cur;
          },
        },
      });
    </script>
  </body>
</html>
